Heretofore, a toner which contains a color developable compound and a color developing agent and is decolorized by heating so that an image formed using the toner can be erased is known. In this technique, a color developable compound and a color developing agent are melt-kneaded along with a binder resin by a kneading pulverization method, thereby incorporating the color developable compound and the color developing agent in the inside of the toner. By heating paper printed using this toner at a temperature between 100° C. and 200° C. for about 1 to 3 hours, the printed region can be decolorized, and further, the decolorized paper can be reused. This technique is an excellent technique capable of contributing to a decrease in the environmental load by reducing the consumption of paper.
Among the decolorizable toners, there is a toner in which a colorant (containing a color developable compound and a color developing agent) is incorporated in a capsule, which has a size of about several micrometers. Meanwhile, also a toner has a size of only about several micrometers to 20 μm. Therefore, if the incorporation of a colorant in the form of a capsule is not sufficient, the colorant is significantly exposed on the surface of a binder resin.
Such a toner is subject to stress such as stirring when used in an image forming apparatus such as MFP and is easily broken at the interface between the binder resin and the colorant in the form of a capsule, and therefore is liable to generate fine powder of the binder resin.
As for the measurement of fine powder, a technique in which the amount of a toner in the form of a fine powder (having a small particle diameter, more specifically, having a largest number particle diameter of from 2 to 4 μm or less) is measured using a flow particle image analyzer and a technique in which after a dispersion liquid containing a toner dispersed therein is irradiated with an ultrasonic wave, particles having a size of from 0.5 μm to 2 μm are measured using a flow particle image analyzer are proposed.
However, in these techniques, only the amount of fine powder of a toner after production is measured. Further, by the irradiation with an ultrasonic wave, the amount of fine powder is liable to increase as compared with a toner after production, however, a stress equivalent to that in a developing device cannot be applied to a toner, and therefore, the amount of fine powder when the toner is actually used cannot be reproduced. Therefore, according to a conventional technique, an effect on an image quality such as fogging or contamination of an apparatus due to toner scattering is not sufficiently improved.